Liquid crystals have been used to display information in flat-panel displays for many years, such as are commonly used in watch faces or half page size displays for lap-top computers and the like.
One current display technology is the super twisted nematic (STN) type. Although these types of devices are relatively inexpensive, they are limited in the number of lines that can be addressed because of the steepness of their electro-optic curve and the tight voltage tolerances that need to be maintained. Even though the technology is now highly developed current devices are limited to approximately 500 lines. These displays also have the drawback of requiring polarizers, which limits brightness, and require glass substrates which increase the weight of the display and are susceptible to breakage. Another current display technology is the so called thin film transistor (TFT) type. In these devices, the liquid crystal electro-optical element is driven by a thin film transistor that is present at each pixel in a so called active matrix. These types of displays are expensive to manufacture. As the size of the existing half page TFT displays is increased to full page size, the number of transistors and the area of substrate increase by a factor of two and, at the current time, this results in an unacceptable expense.
Thus, a principal shortcoming of the current generation of displays is that they are inherently limited to half page, rather than full page capability. Accordingly, there is still a need for a technology that enables cost effective page size displays that can be updated at page turn rates.
Liquid crystal displays made up of bistable chiral nematic materials do not require continuous updating or refreshing. When data or information changes on the display, the electronics update the display. If, however, the display information does not change, the display can be written once and remain in its information-conveying configuration for extended periods without display processor intervention. The ability to remain in a stable state for an extended period has resulted in use of chiral nematic liquid crystal displays for signs that can be slowly updated over relatively long periods of time. Since the display information does not change, the fact that it may take longer to write the initial information to the display is not important.
Advantageously, chiral nematic bistable devices can be prepared that have no limit to the number of lines that can be addressed, making them excellent candidates to provide the needed page size displays. However, the update rate of these displays is far too slow for many page size display applications such as electronic manuals or electronic news papers and the like. For these types of applications, the device needs to be addressed in about 1 second or less to be comparable with the time required to turn a page manually. However, the update refresh rate of the current chiral nematic display technology is greater than 10 seconds for a 1000 line page size display. Clearly, there is a need for a commercially viable display for use in information-conveying devices such as page size viewers, electronic books, pagers and telephone displays, and signs that must be addressed more quickly. Updating information on a passive matrix liquid crystal display at rates fast enough to convey information in a commercially acceptable manner of, for example, 1 second or less has presented a problem to prior art liquid crystal displays.
A number of prior art patents address problems in updating liquid crystal display information. So-called liquid crystal display drivers or electronic circuits are known in the prior art and utilize various techniques for updating a liquid crystal display. U.S. Pat. No. 5,251,048 which issued Oct. 5, 1993 to Doane et al. concerns a method and apparatus for electronic switching of a reflective color display system. This patent discloses use of a liquid crystal light-modulating material that is confined between substrates. Elongated conductive paths supported on opposite sides of the substrates activate picture elements at controlled locations to set up a display screen. The disclosure of the '048 patent to Doane et al. is incorporated herein by reference.
A paper entitled "Storage Type Liquid Crystal Matrix Display" to Tani et al. (SID 79 Digest, p. 114-115) proposes a liquid crystal display driver system whose operation takes into account transitions between various states of a chiral nematic liquid crystal material. The paper describes a new storage type liquid crystal display having the advantages of long storage time which makes refreshing or updating of the information on the display unnecessary. However, the Tani et al. drive scheme is limited in its resolution and information density conveying ability. His drive waveform and technology are limited in the number of lines that can be addressed to roughly 100 lines, far less than the 1000 lines required for page size viewer applications. Also, his demonstrated writing times of greater than 8 ms per line are insufficient for commercially acceptable page size viewers. On a flat-panel display or the like, 100 lines of information in a liquid crystal display is not acceptable for conveying text and 8 ms per line is far too slow for many applications.